In the quality laboratory of a modern paper mill, color, brightness, whiteness, and fluorescence of the product are conventionally measured on a multiple sheet “pad” of paper, rather than on a single sheet. If only a single sheet is measured, the results will be influenced by both the partial transparency of the sheet and the reflectance of the backing against which the sheet is observed. Furthermore, the “infinite pad” value is usually what the end customer is concerned with, since this is typically how the customer will view the end product. However, these measurement conditions cannot necessarily be reproduced in-situ in the manufacturing process, where an “on-line” color sensor can view only a single thickness of the product.
Several strategies have been employed to improve the agreement of on-line color measurements with laboratory “pad” measurements. One strategy, an example of which is disclosed in U.S. Pat. No. 4,715,715, provides a backing to the sheet with an opaque material which approximates the color and optical scattering power of the paper being manufactured. A second strategy is to measure the sheet spectral reflectivity twice, once backed with a highly reflective (i.e., “white”) material, and once backed with a highly absorptive (i.e., “black”) material. From these independent measurements, the spectral transparency can be determined and the infinite pad spectral reflectivity calculated according to the Kubelka-Munk theory. An example of an apparatus for measuring dark and bright reflectances in succession is disclosed in U.S. Pat. No. 4,944,594.
The color of a material is commonly described using colorimetric quantities such as CIE L*a*b* values and auxiliary quantities such as Technical Association of the Pulp and Paper Industry (TAPPI) brightness. These can be computed from the total radiance factor of the material for a particular condition of illumination, together with knowledge of that illumination. For example, the reflective color or transmissive color of a material can be characterized using appropriate measurements of reflective or transmissive total radiance factor. To characterize color reliably, it may be necessary to know the total radiance factor in most or all of the visible range of wavelengths, at least from 420 nm to 650 nm, but typically from 400 nm to 700 nm. These measurements can be made using any particular geometry of illuminator and detector with respect to the measured material, and a number of geometries have been adopted as standards by international bodies.
For a non-fluorescent material, the reflective total radiance factor may always be identical to the reflectance spectrum, and the material's transmissive total radiance factor may always be equal to the material's transmittance. These are invariant under different conditions of illumination, so that a determination of reflectance or transmittance using a single illuminator is sufficient to characterize the corresponding total radiance factor under any other illuminator. Accordingly, it may only be necessary to use a single illuminator in measuring the color of a non-fluorescent material.
However, this may not be true for fluorescent materials, for which the measured total radiance factor generally depends on the illuminator used in the measurement. This is because the total radiance factor may be determined by fluorescent emission as well as by reflection or transmission of incident light. Thus, a total radiance factor measured using one illuminator need not be the same as the total radiance factor measured using a different illuminator, and a measured total radiance factor is generally valid only for the illuminator used in the measurement. For instance, in paper containing stilbene-based fluorescent whitening agents, the total radiance factor at 450 nm will depend on the ratio of the spectral power of the illuminator at 450 nm to the material's spectral power in the excitation band for fluorescent emission at 450 nm, particularly from 330 nm to 420 nm. This issue and the consequences for color measurement are explained in more detail in T. Shakespeare & J. Shakespeare “Problems in colour measurement of fluorescent paper grades”, Analytica Chimica Acta 380(2)227-242, 1999.
A strategy used to measure the color of fluorescent paper is to measure the total radiance factor (which in prior art is sometimes misleadingly referred to as a reflectance factor) using two different illuminators. For example, U.S. Pat. No. 4,699,510 discloses an on-line color sensor for measuring the color of a moving sheet of paper that contains fluorescent whitening agents (FWA). Fluorescent whitening agents typically absorb the violet and ultraviolet energies of incident light and re-emit these energies in the blue range of the visible spectrum to give the paper a whiter appearance. The '510 patent discloses techniques for determining the color spectrum of such treated paper if illuminated by a defined source such as the CIE D65 (North Sky Daylight) standard source. The D65 standard source has an energy distribution which, compared to other standard sources such as CIE source C, is relatively bright in the 300-400 nm range; consequently, paper with fluorescent whitening agents is likely to appear bluer if illuminated by a D65 source.
The color sensor of the '510 patent has two sources of illumination, one an ultraviolet source which emits light primarily in the excitation band of fluorescent whitening agents, the other a visible light source with an emission spectrum approximating a CIE standard source which also emits a significant amount of light in the UV or excitation range of FWA.
However, methods such as those of the '510 patent may be of limited efficacy, in that by using two illuminators, it is possible to reliably determine the total radiance factor only for the range of illuminators which can be formed as linear combinations of the two illuminators used in measurement. An alternative set of methods is disclosed in U.S. Pat. No. 6,263,291 and U.S. Pat. No. 6,272,440 which describe sequential use of plural monochrome or narrow-band illuminators in measurement of color. In this way, the measurement apparatus sequentially determines individual rows of the radiance transfer factor matrix, from which a total radiance factor can be computed for any illuminator. However, these are slow methods of limited reliability, since the devices require extended sequences of measurements with long integration times in each measurement of the sequence, and the devices also demand precise measurements of near-zero light fluxes to characterize the off-diagonal values of the radiance transfer factor matrix. The devices may thus be poorly suited to industrial applications, which may require prompt measurement of single samples, or may require measurement of rapidly moving materials whose color may be varying. For example, in manufacture of paper, the paper sheet may move at speeds approaching 30 meters per second, and exhibit variations in color properties over distances of less than one meter.
An improved approach is disclosed in U.S. patent application Ser. No. 09/957,085 in which plural rich spectral illuminator states are used sequentially, possibly in a random sequence, and a statistical decomposition of spectrophotometric measurements is used to infer the radiance transfer factor matrix. In this approach, an intrinsically unstable light source, such as a Xenon flash tube or some other light source with an unstable power supply is used to ensure spectral variability of the illuminator. Thus, the radiance transfer factor matrix can be determined from a sequence of measurements, but the method does not require long measurement integrations in each measurement nor does the method require particularly precise measurements of small light fluxes. However, the method does require that the entire radiance transfer factor matrix be known from a sequence of measurements in order to compute the total radiance factor for a specific illuminator. This is because it is unlikely that any particular illuminator state used in measurement matches the specified illuminator closely enough for a single measurement to reliably provide its total radiance factor. The method therefore requires a significant time in which to determine the radiance transfer factor, during which time the sample to be measured must be stationary, or if measurement is made of a moving material, the properties of the material must not change over the distance moved during the determination.
In paper and board manufacturing, various machines impart vibration to the environment. These vibrations may shorten the expected life of the illuminating device used in the previously described sensor and similar devices. A short life of the illuminating device may require replacement of the illuminating device which not only incurs costs for replacement but may also incur costs associated with a component of the manufacturing process going off-line while the illuminating device is replaced. In particular, filament-based illuminators such as Tungsten-halogen lamps may be prone to rapid failure in vibration-rich environments, since the filament is fragile and easily disintegrates. Low-pressure discharge tubes, such as Xenon flashtubes, also suffer from shortened service life in such environments, due to the existence of stress concentration points in the bulb material and the likelihood of resonant vibration frequencies.
Accordingly, an efficient and effective device, method, and system are needed for rapid and timely determination of the color of fluorescent and non-fluorescent samples and materials. In addition, the device, system and method may provide an illuminating device that can handle unstable environments with substantial vibration. The device, system and method may provide an illuminating device that provides for efficient measurement of the color of fluorescent material and maintenance of the sensor.